The Environmental Protection Agency (EPA) and Organization for Economic Cooperation and Development (OECD) estimates that as many as 80,000 endocrine disruptor compounds (EDCs) may be evaluated to determine their estrogenic or anti-estrogenic activity. The development of standardized protocols to test for hormonally active ED compounds is a major goal of regulatory agencies worldwide. Also, the role of phytoestrogens present in commercially available rodent diets on reproduction and carcinogenicity endpoints when EDCs are being evaluated requires serious consideration. Our studies were aimed at determining the effects of dietary factors including the phytoestrogen content (daidzein and genistein) and total metabolizable energy (ME) on uterine weights and vaginal opening (VO) endpoints of immature mice and rats. We also assessed VO as an endpoint for selecting the most appropriate diets(s) for comparing the estrogenic or anti-estrogenic activity of EDCs. Pre-pubertal CD-1 mice were weaned at post-natal day (PND) 15 and fed test diets containing predetermined levels of dietary estrogens from PND 15 to PND 22 to determine the effects of dietary phytoestrogens on uterine weights. Mice were fed the test diets from PND 15 to PND 35 or until VO occurs to determine the effects of dietary phytoestrogens on the time of VO. Vaginal opening was recorded daily from weaning to time of VO. The phytoestrogen content of the diet was highly predictive (P less than 0.0001) of the proportion of mice exhibiting VO at PND 24. Total ME was also significantly (P less than 0.01) correlated with the time of VO, although the predictability of this variable was less powerful than the phytoestrogen content. The time of VO in mice was significantly (P less than 0.05) accelerated in mice fed diets high in phytoestrogens as compared to low phytoestrogen content diets. It was concluded that 1) dietary daidzein and genistein can significantly (P less than 0.01) accelerate the time of VO in CD-1 mice, 2) the advancement in the time of VO is a sensitive endpoint for evaluating the estrogenic activity of EDCs, and should be part of the standard protocol for evaluating EDCs, 3) the phytoestrogen content of the same diet can vary 3 to 4 fold, 4) different mill dates of the same PMI 5002 diet produced significant (p less than .05) in the time of VO in CD-1 mice and F344 rats but not in CD Sprague Dawley (SD) rats, and 5) the CD Sprague Dawley rat is less sensitive to dietary phytoestrogen when compared with the CD-1 mouse or the F344 rat suggesting that the SD rat is not the most sensitive model for evaluating the estrogenic activity of EDCs when using the time of VO as the hormonal endpoint. We have recently reported the advantages/disadvantages of open and closed formula laboratory animal diets and their importance to research. In addition, we have reported the endotoxin, coliform and dust levels in the various types of rodent bedding. We concluded that paper bedding is the optimal bedding type for conducting LPS inhalation studies and that rodent bedding containing high levels of endotoxin may alter the results of respiratory and immunologic studies in rodents. We have shown that the estrogenic mycotoxin zearalenone is ubiquitious in commercially available corn-cob bedding. 154 of 189 (84%) of the samples were naturally contaminated with zearalenone at levels ranging form 100 to 7,000 ppb (mean 500 ppb). We have shown that levels of 5-10 ppm can significantly advance the time of VO in immature CD-1 haired and SKH-1 hairless mice. Recent studies have focused on: 1) reviewing the literature on BPA and determining the estrogenic content of rodent diets, bedding, cages and water bottles and its impact on Bisphenol A studies. Our literature review indicated that the results of low dose BPA (50 g /kg bwt/day) animal studies are inconsistent, and the use of high phytoestrogen diets and the subcutaneous route of exposure are contributing factors. Our literature review further indicates that the identification and reporting of the diet, bedding, caging and water bottles used in BPA studies are not consistent. We concluded that a diet containing reduced levels of phytoestrogens (<20 g/g diet) and low levels of metabolizable energy (3.1 k cal/ diet), estrogen-free bedding, cages and water bottles be used for studies evaluating the estrogenic activity of endocrine disrupting compounds such as BPAs. 2) That the oral route of BPA exposure should be used when results are to be extrapolated to determine potential adverse effects in humans. Also, we are determining the effects of BPA and other estrogenic monomers leaching from the various types of animal cages and water bottles and their impact on the time of VO in immature CD-1 mice.